FIG. 16 is a cross-sectional view showing a rotational tool retaining member unit 1 according to a first related art. A friction stir welding gun (hereinafter referred to as “FSW gun”) which is a friction stir welding apparatus according to the related art is mounted to a tip end portion of a robot by a gun adapter. The FSW gun has a C-shaped support frame. A fixed tool retaining member unit which is not shown is provided at one end portion of the support frame, while the rotational tool retaining member unit 1 is provided at the other end portion of the support frame.
The rotational tool retaining member unit 1 serves to retain a rotational tool 2. The rotational tool retaining member unit 1 includes a rotary base 3 which is fixed to a drive shaft of the FSW gun and has a tapered inner peripheral surface, a chuck member 4 which is fittingly retained by the rotary base 3 and has a tapered outer peripheral surface, and an operation member 5 which is detachably coupled to the chuck member 4 and is threadedly engaged with the rotary base 3. By rotating and axially displacing the operation member 5 relative to the rotary base 3, the inner diameter of the chuck member 4 is changed.
The rotational tool retaining member unit 1 retains the rotational tool 2 by inserting a cylindrical shaft portion 6 of the rotational tool 2 into the chuck member 4 and by rotating the operation member 5 by a hand operation to reduce the inner diameter of the chuck member 4. In such a retained state, the rotational tool 2 rotates together with the rotary base 3, so that the rotational tool 2 joined to materials can be pulled out from the materials after the friction stir welding.
The rotational tool retaining member unit 1 is configured to removably mount the rotational tool 2 by the hand operation of the operation member 5. The rotational tool 2 is changeable, for example, when it has worn out (see Japanese Laid-Open Patent Application Publications Nos. 2001-314982 and 2002-137067).
In the friction stir welding, the rotational tool 2 needs to be changed depending on the shape and quality of a portion of the materials. Changing the rotational tool 2 by the hand operation described about the first related art significantly reduces efficiency. Therefore, development of a tool changer which is capable of changing the rotational tool 2 automatically by a mechanical operation has been demanded. One example of the tool chamber may be application of a device which is capable of automatically changing a grip mounted to a robot hand at the tip end of the robot according to a second related art by a mechanical operation.
FIG. 17 is a cross-sectional view showing a robot hand 10 and a grip 15 according to the second related art. The robot hand 10 includes a vertical feed shaft 11 of a cylindrical shape, a rotational shaft 12 which is of a bottomed cylinder shape and is rotatably accommodated inside the vertical feed shaft 11, and a grip presser 14 which is accommodated inside the rotational shaft 12 and is subjected to a downward force exerted by a coil spring 13. The rotational shaft 12 opens downward and is provided at a lower end thereof with a plurality of hook-shaped grooves 12a. 
The grip 15 has a shank 16 fitted into the rotational shaft 12. A plurality of rollers 17 are provided on the peripheral surface of the shank 16 to respectively correspond to the plurality of grooves 12a. A plate 18 is provided at a lower end of the shank 16. The plate 18 is mounted to an accommodating base 19 in such a way that the plate 18 is positioned by positioning pins 19a provided on the accommodating base 19.
When mounting the grip 15 to the robot hand 10, the vertical feed shaft 11 is first moved down and the shank 16 is inserted into the rotational shaft 12 while inserting the rollers 17 into openings of grooves 12a, respectively. Then, by the up-down movement of the vertical feed shaft 11 and the rotation of the rotational shaft 12, the rollers 17 are moved toward the tip end portions of the grooves 12 and fitted thereinto. The rollers 17 fitted into the tip end portions are inhibited from coming off the tip end portions, because the shank 16 is pressed down by the grip presser 14. In this manner, the grip 15 is automatically mounted to the robot hand 10 (see Japanese Utility Model Application Publication No. 55-151489).
In the friction stir welding, it is required that the materials be stirred while pressing the rotational tool 2 against the materials via the rotary base 3. The pressing force is controlled at a desired value by a controller which is not shown. When the mounting structure according to the second related art is applied to the friction stir welding apparatus according to the first related art, the rotational tool retaining member unit 1 presses down the rotational tool 2 corresponding to the grip 15 via the grip presser 14. For this reason, the pressing force exerted by the rotational tool retaining member unit 1 to the rotational tool 2 is absorbed by the coil spring 14. In order to press the rotational tool 2 with a desired pressing force, a biasing force exerted on the grip presser 14 must be taken into account. If the biasing force exerted on the grip presser 14 is taken into account, the control for the pressing force exerted by the rotational tool 2 to the materials is complex.